1. Field of the Invention
The present invention relates to an imperfect register correcting method to be carried out on a multicolor image forming apparatus having a plurality of image forming units capable of sequentially transferring color images in register in a superposed arrangement on a recording medium to form a multicolor image on the recording medium.
2. Description of the Prior Art
The use of color documents in offices has rapidly become prevalent in recent years and color image forming apparatus for producing color documents, including color copying machines, color printers and color facsimile equipments have rapidly become widespread. The enhancement of the operating speed of the color image forming apparatus is the trend of the times.
A so-called tandem color printer is an example of a high-speed color image forming apparatus. The tandem color printer is provided with four ROSs (raster output scanners), i.e., image forming units, respectively for forming, for example, an yellow (Y) image, a magenta (M) image, a cyan (C) image and a black (K) image. The tandem color printer superposes four color images formed sequentially by the ROSs to form a multicolor image efficiently. When the tandem color printer jams or malfunctions, some of the ROSs are removed or displaced, recording sheets clogging the tandem color printer are removed or faulty parts are repaired or replaced, and then the removed or displaced ROSs are restored to their original positions. However, the ROSs are not necessarily restored correctly to their original positions. If the ROSs are dislocated from the correct positions, the superposed color images are imperfectly registered.
Factors that cause imperfect register are dislocation of the ROS with respect to the scanning direction, dislocation of the ROS with respect to the feed direction, i.e., the direction of advancement of the recording sheet, incorrect scanning width, i.e., deviation of the image in size with respect to the scanning direction, angular deviation of the scanning direction, i.e., skew of the ROS.
An imperfect register correcting method is disclosed in Japanese Patent Laid-open (Kokai) No. Hei. 1-142671. According to this known imperfect register correcting method, a register mark is formed by each of a plurality of ROSs, the register mark generator of each ROS generates the image of the register mark according to a specific rule to form the image of the register mark on the conveyor belt, the image of the register mark is sampled at predetermined moments by CCD image sensors to obtain register mark data. Therefore, the positional differences of the register marks formed by the ROSs containing such as dislocations of the ROSs with respect to the scanning direction, dislocations of the ROSs with respect to the feed direction, incorrect scanning width of the ROSs and skews of the ROSs are corrected at once on the basis of the calculated positional differences between the register marks.
However, when the skew is corrected, at least one of the factors of imperfect register, i.e., dislocation with respect to the scanning direction, dislocation with respect to the feed direction and incorrect scanning width, changes. Therefore, if the changed factor is corrected on the basis of corrections calculated on the basis of the register mark data sampled before the correction of the skew, the factor cannot be corrected exactly and hence perfect correction of imperfect register is impossible; that is, since secondary errors are introduced additionally into the factors of imperfect register by the correction of the skew, the secondary errors remain uncorrected even if the factors are corrected on the basis of the corrections calculated on the basis of the register mark data sampled before the correction of the skew.
The secondary errors will be described concretely with reference to FIGS. 5 and 6 showing the images of two register marks in terms of the effect of skew correction on the dislocation of the images with respect to the feed direction. Referring to FIG. 5, a first image 100a, i.e., the image of a first register mark, and a second image 101a, i.e., the image of a second register mark, are formed on the basis of the image data obtained by sampling. The second image 101a is skewed at a skew angle with respect to the first image 100a and dislocated by a dislocation 102a from the first image 100a. If the skew angle of the second image 101a is corrected, the second image is shifted to a first corrected position 101a'. The second image 101a at the first corrected position 101a' is dislocated with respect to the feed direction by a dislocation 102b from the first image 100a. Thus, the dislocation 102b of the second image 101a at the first corrected position 101a' from the first image 100a with respect to the feed direction differs considerably from the dislocation 102a of the second image 101a from the first image 100a calculated on the basis of the image data sampled before skew correction. If the dislocation of the second image 101a at the first corrected position 101a' is corrected subsequent to skew correction on the basis of a correction determined on the basis of the dislocation 102a calculated on the basis of the image data sampled before skew correction, the second image 101a is shifted to a second corrected position 101a" as shown in FIG. 6, in which the second image 101a is still dislocated with respect to the feed direction from the first image 100a by a dislocation equal to the difference between the dislocations 102a and 102b. Thus, the positional error of the second image 101a with respect to the feed direction cannot be correctly corrected if the dislocation of the second image 101a with respect to the feed direction is corrected after skew correction on the basis of the dislocation 102a calculated on the basis of the image data sampled before skew correction. This known imperfect register correcting method, which corrects the factors of imperfect register on the basis of the image data sampled before skew correction, is unable to precisely correct other factors of imperfect register as well as the dislocation with respect to the feed direction.
Even if a skew correcting mechanism capable of correcting the skew of an image without affecting other factors of imperfect register is available, the skew correcting mechanism will be very complex and very expensive.